Schottky barrier rectifiers are extensively used as output rectifiers in switching mode power supplies and in other power applications for carrying large currents at high voltages. As is well known to those having skill in the art, a rectifier exhibits a very low resistance to current flow in one direction and a very high resistance to current flow in the opposite direction. As is also well known to those having skill in the art, a Schottky barrier rectifier produces rectification as a result of nonlinear current transport across a metal-semiconductor contact.
A Schottky barrier rectifier power device, for carrying large currents at high voltages, typically contains a relatively large active area where the metal-semiconductor contact is made. The Schottky barrier height in the active area is determined by the metal, and is the same over the entire active area. In a Schottky power rectifier, the dominant current flow is by thermionic emission. Reverse blocking takes place by introducing a depletion layer into the semiconductor substrate.
Two important device characteristics for the Schottky barrier rectifier are the on-state (or forward) voltage drop and the reverse leakage current. The on-state voltage drop is the voltage across the rectifier during forward current conduction. As the power supply voltage for modern electronic devices continues to decrease, it is desirable to decrease the on-state voltage drop of the device. The reverse leakage current is the leakage current which flows through the device when it is in blocking mode. In order to reduce the power dissipation during reverse blocking mode, it is desirable to decrease the reverse leakage current.
Unfortunately, there is a tradeoff between the forward voltage drop and the reverse leakage current in a Schottky barrier rectifier, so that it is difficult to minimize both characteristics simultaneously. In particular, as the Schottky barrier height is reduced, the forward voltage drop decreases but the leakage current increases. Conversely, as the barrier height is increased, the forward voltage drop increases but the leakage current decreases.
In designing Schottky barrier rectifiers, a barrier height is selected based on the intended device application. Low barrier heights are typically used for Schottky rectifiers intended for high current operation with large duty cycles, where the power losses during forward conduction are dominant. Larger barrier heights are typically used for Schottky rectifiers intended for applications with higher reverse bias stress and higher ambient temperatures.
The Schottky barrier height is related to the work function of the metal used in the Schottky barrier rectifier. A graphical illustration of the relationship between metal work function and Schottky barrier height may be found in Chapter 5, FIG. 3 of the textbook by S. M. Sze entitled Semiconductor Devices, Physics and Technology, John Wiley & Sons, 1985, at page 163. As shown, the following sequence of metals exhibit increasing work functions and increasing barrier heights: magnesium (Mg), hafuium (Hf), Aluminum (Al), Tungsten (W), palladium (Pd) and platinum (Pt). Among the most commonly used metals, platinum and molybdenum barrier devices exhibit the best behaved forward conduction characteristics. The leakage current of tungsten Schottky barrier rectifiers is lower, but their forward voltage drop is greater than for platinum and molybdenum devices.
The desired barrier height for Schottky power rectifiers may be obtained by selecting the appropriate metal to form the Schottky barrier. However, it is well known by those having skill in the art that the effective barrier height can also be altered by a very shallow ion implant, typically less than 100.ANG. thick, at the surface of the semiconductor substrate, with a carefully controlled dose. For an n-type semiconductor substrate, an n-type implant layer will lower the barrier height, whereas a p-type layer will raise it. Barrier altering implants are often used because they allow selection of the metal based on the metallurgical properties of the interface which will produce the most reliable operation, while allowing tailoring of the barrier height by controlling the ion implant dose. Optimization of the Schottky barrier can be achieved by starting with a larger Schottky barrier height and lowering it with an implant layer. Optimization of a Schottky barrier can also be achieved by starting with a low Schottky barrier height metal and raising it with an implant layer.
It is important to maintain the implanted charge close to the surface, adjacent the metal. Antimony implantation at energies of 5-10 keV is effective for accomplishing barrier height altering in n-type silicon because its larger mass results in a shallow implantation depth, and its low diffusion coefficient prevents redistribution during subsequent implant activation or other high temperature processing steps. A detailed and comprehensive discussion of the design of Schottky barrier power rectifiers may be found in Section 8.2 of the textbook entitled Modern Power Devices by coinventor B. J. Baliga, published by John Wiley and Sons, Inc., 1987, the disclosure of which is hereby incorporated herein by reference.
One attempt to reduce the on-state voltage drop/reverse leakage current tradeoff of the Schottky barrier rectifier is the Junction Barrier controlled Schottky (JBS) rectifier. The JBS rectifier is a Schottky rectifier structure with a p-n junction grid integrated into its semiconductor substrate. This device structure is also called a "pinch" rectifier. The junction grid is designed so that its depletion layers do not pinch-off under zero and forward bias conditions. The device thereby contains multiple conductive channels under the Schottky barriers through which current can flow during forward bias operation.
Under reverse bias, the depletion layers formed at the p-n junctions spread into the channel under the Schottky barriers. The junction grid is designed so that the depletion layers intersect under the Schottky barrier when the reverse bias exceeds a few volts, to thereby pinch-off the conductive channels. After depletion layer pinch-off, a potential barrier is formed in the channel, and further increases in applied voltage are supported by the depletion layer extending away from the Schottky barrier. The potential barrier shields the Schottky barrier from the applied voltage. This shielding prevents Schottky barrier lowering and eliminates the large increase in leakage current for conventional Schottky rectifiers.
Because of this suppressed leakage current, the Schottky barrier used in the JBS rectifier can be significantly less than that for conventional Schottky rectifiers. This has allowed a reduction in the forward voltage drop while maintaining an acceptable reverse blocking characteristic. The design and operation of the JBS rectifier is described in Section 8.4 of the above cited textbook by coinventor Baliga. Notwithstanding this development, there continues to be a need for a Schottky barrier rectifier which minimizes the on-state voltage drop and the reverse leakage current.